xen: move Xen-testing predicates to common header

Move xen_domain and related tests out of asm-x86 to xen/xen.h so they
can be included whenever they are necessary.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
diff --git a/drivers/video/xen-fbfront.c b/drivers/video/xen-fbfront.c
index 54cd916..966b226 100644
--- a/drivers/video/xen-fbfront.c
+++ b/drivers/video/xen-fbfront.c
@@ -25,7 +25,10 @@
 #include <linux/module.h>
 #include <linux/vmalloc.h>
 #include <linux/mm.h>
+
 #include <asm/xen/hypervisor.h>
+
+#include <xen/xen.h>
 #include <xen/events.h>
 #include <xen/page.h>
 #include <xen/interface/io/fbif.h>